Call Stored Procdure in entity frame work

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MVC5

stored-procedure

I have problem n entity frame work .

I do not Know how use stored procdure in entity frame work .

for example

this code

 private FidilioEntities _db;

public string GetGoldenMember()
 {
     var res = _db.GoldenMemebrCustomer().FirstOrDefault();
     string json = JsonConvert.SerializeObject(res);
     return json;

 }

GoldenMember Stored procdure Return me list of people . but when i use it i have this error :
Object reference not set to an instance of an object


or in this code :

var tt = new UserActivityVM();
      var res = _db.UsersAvtivityReport(type).FirstOrDefault();
      string jso = JsonConvert.SerializeObject(res);

      tt.UsersActivity = jso;
      return tt.UsersActivity;

UsersAvtivityReport return me only one record but same as other i Have this error
Object reference not set to an instance of an object

where is the problem ?